Process for the preparation of middle distillates

ABSTRACT

A process for the preparation of one or more hydrocarbon fuel products boiling in the kero/diesel range from a stream of hydrocarbons produced in a Fischer-Tropsch process, in which process synthesis gas is converted into liquid hydrocarbons, at least a part of the hydrocarbons boiling above the kero/diesel range, having the following steps:
         (1) hydrocracking/hydroisomerizing at least a part of the Fischer-Tropsch hydrocarbons stream at a conversion per pass of at most 80 wt % of the material boiling above 370° C. into material boiling below 370° C.;   (2) separating the product stream obtained in step (1) into one or more light fractions boiling below the kero/diesel boiling range, one or more fractions boiling in the kero/diesel boiling range and a heavy fraction boiling above the kero/diesel boiling range;   (3) hydrocracking/hydroisomerizing the major part of the heavy fraction obtained in step (2) at a conversion per pass of at most 80 wt % of the material boiling above 370° C. into material boiling below 370° C.;   (4) separating the product stream obtained in step (3) into one or more light fractions boiling below the kero/diesel boiling range, one or more fractions boiling in the kero/diesel boiling range and a heavy fraction boiling above the kero/diesel boiling range; and,   (5) hydrocracking/hydroisomerizing the major part of the heavy fraction obtained in step (4) in the hydrocracking/hydroisomerizing process described in step (1) and/or step (3), in which process the Fischer-Tropsch hydrocarbons stream comprises at least 35 wt % C 30 + (based on total amount of hydrocarbons in the Fischer-Tropsch hydrocarbons stream) and in which stream the weight ratio C 60 +/C 30 + is at least 0.2.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of one ormore hydrocarbon fuel products boiling in the kero/diesel range from astream of hydrocarbons produced in a Fischer-Tropsch process and tohydrocarbons so produced.

BACKGROUND OF THE INVENTION

Today the energy requirements of the transport sectors are dominated byliquid fuels derived from the fractionation and processing of crude oil.The dominance of liquid fuels is expected to continue.

Crude oils derived from liquid fuels usually are not clean. Theytypically contain significant amounts of sulphur, nitrogen andaromatics. Diesel fuels derived from crude oil show relatively lowcetane values. Clean distillate fuels can be produced from petroleumbased distillates through (severe) hydrotreatment at great expense. Fordiesel fuels, however, these treatments usually hardly improve thecetane number.

Another source for distillate fuels, especially middle distillates, i.e.kerosene and diesel, is the Fischer-Tropsch process, especially usingcobalt catalysts. During the last two decades this process has evolvedas a key process for the conversion of natural gas into especiallymiddle distillates of high quality. In this process synthesis gas isconverted in several steps into middle distillates. First, natural gasin converted into synthesis gas by means of a (catalytic) partialoxidation process and/or steam reforming process. In a second step thesynthesis gas is converted into long chain paraffins (the average C₅+hydrocarbon usually comprising 25 to 35 carbon atoms). In a third stepthe long chain hydrocarbons are hydrocracked into molecules of thedesired middle distillate fuels. In this respect reference is made to EP161 705, EP 583 836, EP 532 116, WO 99/01218, U.S. Pat. No. 4,857,559and EP 1 004 746. Further reference is made to H M H van Wechem and M MG Senden, Conversion of Natural Gas to Transportation Fuels, Natural GasConversion II, H E Curry-Hyde and R F Howe (editors), Elsevier ScienceB.V. pages 43-71.

In general, the quality of the middle distillates prepared by theFischer-Tropsch process is excellent. The mainly paraffinic products arefree from sulphur, nitrogen and aromatic compounds. The kerosene anddiesel have excellent combustion properties (smoke point and cetanenumber). The cold flow properties meet the relevant specifications. Ifnecessary, additives may be used to meet the most stringent cold flowspecifications. In addition, also the usual additives may be added.

In view of the continuously increasing requirements of the middledistillate properties, there is a need to further improve the middledistillate properties, especially the cold flow properties of the middledistillates. Thus, there is a need for middle distillates with improvedintrinsic cold flow properties, i.e. these properties are to be obtainedwithout using any further treatment of the fuels (e.g. dewaxing) orwithout the use of any additives. In addition, for the diesel fractionit is desired that T95, the temperature at which 95 vol % amount ofdiesel boiling, is 380° C. or less, preferably 370° C. or less, morepreferably 360° C. or less, the density (15° C.) should be 840 kg/m³ orless, preferably 800 kg/m³ or less, more preferably 780 kg/m³ or lessand the amount of (poly)aromatic compounds should be zero.

SUMMARY OF THE INVENTION

It has now been found that hydrocracking/hydroisomerising a relativelyheavy Fischer-Tropsch hydrocarbon product (a C₅+ product, preferably aC₁₀+ product) at a relatively low conversion per pass rate, i.e. lessthan 80% conversion of a fraction boiling above a certain boiling point(e.g. 370° C.) which is fed into the reactor into a fraction boilingbelow that boiling point, and subjecting most of the material boilingabove the kero/diesel boiling range to a second, similarhydrocracking/hydroisomerising reaction followed by a recycle of themain part of the material boiling above the kero/diesel boiling range toa hydrocracking/hydroisomerising reaction, results in middle distillatesshowing exceptionally good cold flow properties, making any furthertreatment (to improve the cold flow properties) and/or the use ofadditives in principle superfluous. Compared with Fischer-Tropschproduct which is less heavy (for example the amount of C₃₀+ is e.g. 10%wt less) the cold flow properties (pour point, CFPP) may be 5 or even10° C. better. In addition, T95, density and (poly)aromatic contentsatisfy the ranges as mentioned above. The process is preferably carriedout in a continuous way.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention results in middle distillateshaving exceptionally good cold flow properties. These excellent coldflow properties could perhaps be explained by the relatively high ratioiso/normal and especially the relatively high amount of di- and/ortrimethyl compounds. Nevertheless, the cetane number of the dieselfraction is more than excellent at values far exceeding 60, often valuesof 70 or more are obtained. In addition, the sulphur content isextremely low, always less than 50 ppmw, usually less than 5 ppmw and inmost case the sulphur content is zero. Further, the density ofespecially the diesel fraction is less than 800 kg/m³, in most cases adensity is observed between 765 and 790 kg/m³, usually around 780 kg/m³(the viscosity for such a sample being about 3.0 cSt). Aromaticcompounds are virtually absent, i.e. less than 50 ppmw, resulting invery low particulate emissions. The polyaromatic content is even muchlower than the aromatic content, usually less than 1 ppmw. T95, incombination with the above properties, is below 380° C., often below350° C.

The process as described above results in middle distillates havingextremely good cold flow properties. For instance, the cloud point ofany diesel fraction is usually below −18° C., often even lower than −24°C. The CFPP is usually below −20° C., often −28° C. or lower. The pourpoint is usually below −18° C., often below −24° C.

Due to the relatively heavy Fischer-Tropsch product which is used in theprocess, the overall conversion of the process is extremely high. Thisholds for the carbon conversion as well as for the thermal conversion.The carbon conversion for the Fischer-Tropsch process and thehydrocracking/hydro-isomerising reaction is above 80%, preferably above85%, more preferably above 90%. The thermal conversion for the processwill be above 70%, preferably is above 75%, more preferably is above80%. It is an extremely advantageous situation that such highconversions can be coupled with the extremely good product properties.In addition, the selectivity to C₅+ hydrocarbon is usually above 85 wt%, preferably above 90 wt %, of all hydrocarbons made in theFischer-Tropsch process.

The kero/diesel boiling range in general may vary slightly, depending onlocal conditions, availability of specific feed streams and specificpractices in refineries, all well known to the man skilled in the art.For the purposes of this specification the kero/diesel boiling rangesuitably has an initial boiling point between 110 and 130° C.,preferably at least 140, more preferably at least 150° C., still morepreferably at least 170° C. The final boiling point for the purposes ofthis specification is suitably between 400 and 410° C., preferably atmost 390° C., more preferably at most 375° C., still more preferably atmost 360° C. The end of the kerosene boiling range may be up to 270° C.,usually up to 250° C., but may also be up to 220° C. or even 200° C. Thestart of the diesel boiling range may be 150° C., is usually 170° C. butmay also be 190° C. or even above 200° C. The 50% recovered temperatureof the diesel fraction is preferably between 255 and 315° C., preferablybetween 260 and 300° C., more preferably around 285° C.

It will be appreciated that the one or more hydrocarbon fuel products ofthe present invention suitable is a full range boiling product in thediesel/kero range as defined above, but also very suitably may be twofractions, one boiling in the diesel range, the other boiling in thekerosene range. In addition, three or more fractions, for instance akerosene fraction, a light diesel fraction and a heavy diesel fraction,may be considered as a commercially attractive option. In principle, thenumber of fractions and the boiling ranges will be determined byoperational and commercial conditions.

The synthesis gas to be used for the Fischer-Tropsch reaction is madefrom a hydrocarbonaceous feed, especially by partial oxidation and/orsteam/methane reforming. The hydrocarbonaceous feed is suitably methane,natural gas, associated gas or a mixture of C₁₋₄ hydrocarbons,especially natural gas.

To adjust the H₂/CO ratio in the syngas, carbon dioxide and/or steam maybe introduced into the partial oxidation process. The H₂/CO ratio of thesyngas is suitably between 1.3 and 2.3, preferably between 1.6 and 2.1.If desired, (small) additional amounts of hydrogen may be made by steammethane reforming, preferably in combination with the water gas shiftreaction. The additional hydrogen may also be used in other processes,e.g. hydrocracking.

In another embodiment the H₂/CO ratio of the syngas obtained in thecatalytic oxidation step may be decreased by removal of hydrogen fromthe syngas. This can be done by conventional techniques as pressureswing adsorption or cryogenic processes. A preferred option is aseparation based on membrane technology. Part of the hydrogen may beused in the hydrocracking step of especially the heaviest hydrocarbonfraction of the Fischer-Tropsch reaction.

The synthesis gas obtained in the way as described above, usually havinga temperature between 900 and 1400° C., is cooled to a temperaturebetween 100 and 500° C., suitably between 150 and 450° C., preferablybetween 300 and 400° C., preferably under the simultaneous generation ofpower, e.g. in the form of steam. Further cooling to temperaturesbetween 40 and 130° C., preferably between 50 and 100° C., is done in aconventional heat exchanger, especially a tubular heat exchanger. Toremove any impurities from the syngas, a guard bed may be used.Especially to remove all traces of HCN and/or NH₃ specific catalysts maybe used. Trace amounts of sulphur may be removed by an absorptionprocess using iron and/or zinc oxide.

The purified gaseous mixture, comprising predominantly hydrogen, carbonmonoxide and optionally nitrogen, is contacted with a suitable catalystin the catalytic conversion stage, in which the normally liquidhydrocarbons are formed.

The catalysts used for the catalytic conversion of the mixturecomprising hydrogen and carbon monoxide into hydrocarbons are known inthe art and are usually referred to as Fischer-Tropsch catalysts.Catalysts for use in this process frequently comprise, as thecatalytically active component, a metal from Group VIII of the PeriodicTable of Elements. Particular catalytically active metals includeruthenium, iron, cobalt and nickel. Cobalt is a preferred catalyticallyactive metal in view of the heavy Fischer-Tropsch hydrocarbon which canbe made. As discussed before, preferred hydrocarbonaceous feeds arenatural gas or associated gas. As these feedstocks usually results insynthesis gas having H₂/CO ratio's of about 2, cobalt is a very goodFischer-Tropsch catalyst as the user ratio for this type of catalysts isalso about 2.

The catalytically active metal is preferably supported on a porouscarrier. The porous carrier may be selected from any of the suitablerefractory metal oxides or silicates or combinations thereof known inthe art. Particular examples of preferred porous carriers includesilica, alumina, titania, zirconia, ceria, gallia and mixtures thereof,especially silica, alumina and titania.

The amount of catalytically active metal on the carrier is preferably inthe range of from 3 to 300 pbw per 100 pbw of carrier material, morepreferably from 10 to 80 pbw, especially from 20 to 60 pbw.

If desired, the catalyst may also comprise one or more metals or metaloxides as promoters. Suitable metal oxide promoters may be selected fromGroups IIA, IIIB, IVB, VB and VIB of the Periodic Table of Elements, orthe actinides and lanthanides. In particular, oxides of magnesium,calcium, strontium, barium, scandium, yttrium, lanthanum, cerium,titanium, zirconium, hafnium, thorium, uranium, vanadium, chromium andmanganese are very suitable promoters. Particularly preferred metaloxide promoters for the catalyst used to prepare the waxes for use inthe present invention are manganese and zirconium oxide. Suitable metalpromoters may be selected from Groups VIIB or VIII of the PeriodicTable. Rhenium and Group VIII noble metals are particularly suitable,with platinum and palladium being especially preferred. The amount ofpromoter present in the catalyst is suitably in the range of from 0.01to 100 pbw, preferably 0.1 to 40, more preferably 1 to 20 pbw, per 100pbw of carrier. The most preferred promoters are selected from vanadium,manganese, rhenium, zirconium and platinum.

The catalytically active metal and the promoter, if present, may bedeposited on the carrier material by any suitable treatment, such asimpregnation, kneading and extrusion. After deposition of the metal and,if appropriate, the promoter on the carrier material, the loaded carrieris typically subjected to calcination. The effect of the calcinationtreatment is to remove crystal water, to decompose volatiledecomposition products and to convert organic and inorganic compounds totheir respective oxides. After calcination, the resulting catalyst maybe activated by contacting the catalyst with hydrogen or ahydrogen-containing gas, typically at temperatures of about 200 to 350°C. Other processes for the preparation of Fischer-Tropsch catalystscomprise kneading/mulling, often followed by extrusion,drying/calcination and activation.

The catalytic conversion process may be performed under conventionalsynthesis conditions known in the art. Typically, the catalyticconversion may be effected at a temperature in the range of from 150 to300° C., preferably from 180 to 260° C. Typical total pressures for thecatalytic conversion process are in the range of from 1 to 200 barabsolute, more preferably from 10 to 70 bar absolute. In the catalyticconversion process especially more than 75 wt % of C₅+, preferably morethan 85 wt % C₅+ hydrocarbons are formed. Depending on the catalyst andthe conversion conditions, the amount of heavy wax (C20+) may be up to60 wt %, sometimes up to 70 wt %, and sometimes even up till 85 wt %.Preferably a cobalt catalyst is used, a low H₂/CO ratio is used(especially 1.7, or even lower) and a low temperature is used (190-240°C.), optionally in combination with a high pressure. To avoid any cokeformation, it is preferred to use an H₂/CO ratio of at least 0.3. It isespecially preferred to carry out the Fischer-Tropsch reaction undersuch conditions that the ASF-alpha value (Anderson-Schulz-Flory chaingrowth factor), for the obtained products having at least 20 carbonatoms, is at least 0.925, preferably at least 0.935, more preferably atleast 0.945, even more preferably at least 0.955. Preferably theFischer-Tropsch hydrocarbons stream comprises at least 40 wt % C₃₀+,preferably 50 wt %, more preferably 55 wt %, and the weight ratioC₆₀+/C₃₀+ is at least 0.35, preferably 0.45, more preferably 0.55.

Preferably, a Fischer-Tropsch catalyst is used, which yields substantialquantities of paraffins, more preferably substantially unbranchedparaffins. A most suitable catalyst for this purpose is acobalt-containing Fischer-Tropsch catalyst. Such catalysts are describedin the literature, see e.g. AU 698392 and WO 99/34917 both are herebyincorporated by reference.

The Fischer-Tropsch process may be a slurry FT process or a fixed bed FTprocess, especially a multitubular fixed bed.

The term “middle distillates”, as used herein, is a reference tohydrocarbon mixtures of which the boiling point range correspondssubstantially to that of kerosene and diesel fractions obtained in aconventional atmospheric distillation of crude mineral oil.

Any normally liquid Fischer-Tropsch hydrocarbons mentioned in thepresent description are in general C₅₋₁₈ hydrocarbons or mixturesthereof, although certain amounts of C₄− or C₁₉+ hydrocarbons may bepresent. These hydrocarbons or mixtures thereof are liquid attemperatures between 5 and 30° C. (1 bar), especially at 20° C. (1 bar),and are paraffinic of nature, although considerable amounts of olefinsand/or oxygenates may be present. Suitably up to 20 wt %, preferably upto 10 wt %, of either olefins or oxygenated compounds may be present.Any heavy Fischer-Tropsch wax comprises all hydrocarbons or mixturesthereof which are solid at 20° C., especially C₁₈₋₃₀₀, more especiallyC₁₉₋₂₅₀. Any normally gaseous Fischer-Tropsch hydrocarbons are C₁ to C₄hydrocarbons, although small amounts of C₅+ may be present.

The Fischer-Tropsch step of the present process is followed by a step inwhich at least part of the heavy paraffins-containing hydrocarbonmixture produced in the first step is hydrocracked and hydroisomerized.In this step a catalyst is used which preferably contains acatalytically active metal component as well as an acidic function. Themetal component can be deposited on any acid carrier having cracking andisomerisation activity, for example a halogenated (e.g. fluorided orchlorided) alumina or zeolitic carrier or an amorphous silica/aluminacarrier.

The catalyst used in the hydrocracking/hydroisomerising step of theprocess according to the invention may contain as catalytically activemetal components one or more metals selected from Groups VIB, VIIBand/or VIII of the Periodic System. Examples of such metals aremolybdenum, tungsten, rhenium, the metals of the iron group and themetals of the platinum and palladium groups. Catalysts with a noblemetal as catalytically active metal component generally contain 0.05-5parts by weight and preferably 0.1-2 parts by weight of metal per 100parts by weight of carrier material. Very suitable noble metals arepalladium and platinum. Catalysts with a non-noble metal or acombination of non-noble metals as catalytically active metal componentgenerally contain 0.1-35 parts by weight of metal or combination ofmetals per 100 parts by weight of carrier material. Very suitablehydrocracking catalysts contain a combination of 0.5-20 parts by weightand in particular 1-10 parts by weight of a non-noble metal of GroupVIII and 1-30 parts by weight and in particular 2-20 parts by weight ofa metal of Group VIB and/or VIIB per 100 parts by weight of carriermaterial. Particularly suitable metal combinations are combinations ofnickel and/or cobalt with tungsten and/or molybdenum and/or rhenium.Likewise very suitable as hydrocracking catalysts are catalysts whichcontain 0.1-35 parts by weight and in particular 1-15 parts by weight ofnickel per 100 parts by weight of carrier material.

If the present hydrocracking catalysts contain a non-noble metal orcombination of non-noble metals as catalytically active metal component,they are preferably used in their sulphidic form. The conversion of thehydrocracking catalysts to their sulphidic form can very suitably becarried out by contacting the catalysts at a temperature below 500° C.with a mixture of hydrogen and hydrogen sulphide in a volume ratio of5:1 to 15:1. The conversion of the catalysts into the sulphidic form mayalso be carried out by adding to the feed, under reaction conditions,sulphur compounds in a quantity of from 10 ppmw to 5% by weight and inparticular in a quantity of from 100 ppmw to 2.5% by weight.

The isomerisation/hydrocracking step (2) or (5) of the present processmay be carried out using a catalyst comprising a zeolite having a porediameter in the range from 0.5 to 1.5 Å. The silica:alumina ratio of thezeolite is preferably in the range from 5 to 200. A very suitablecarrier is a mixture of two refractory oxides, especially an amorphouscomposition as amorphous silica/alumina.

The metals can be applied to the carrier in any conventional manner suchas by impregnation, percolation or ion exchange. After the catalyticallyactive metal components have been applied to the carrier, the catalystis usually dried and subsequently calcined. Hydroconversion catalystsare usually employed in the form of particles with a diameter of 0.5-5mm. However, zeolites suitable for use as carrier material for thepresent hydroconversion catalysts are often available as a fine powder.The zeolites may be shaped into particles of larger dimensions, forexample, by compression and extrusion. During shaping the zeolite may,if desired, be combined with an inorganic matrix or binder. Examples ofsuitable matrices or binders are natural clays and synthetic inorganicoxides.

Suitable conditions for the hydrocracking/isomerisation step (1) of theheavy paraffins-containing hydrocarbon mixture according to the processaccording to the invention are a temperature of 280-400° C., preferably290-375° C., more preferably 300-350° C., a pressure between 15 and 200bar, preferably 20-80 bar, more preferably between 20-50 bar, an hourlyspace velocity of 0.2-20 kg of hydrocarbon feed per kg of catalyst perhour, preferably between 0.5 and 3 kg/h, more preferably between 1 and2.5 kg/h, and a hydrogen/hydrocarbon feed molar ratio of 1-50.

The hydrocracking/isomerisation step (1) is preferably carried out insuch a way that the conversion per pass of the material boiling above370° C. (feed plus recycle) into material boiling below 370° C. isbetween 30 and 70 wt %, preferably between 40 and 60 wt %, morepreferably about 50 wt %.

Suitably at least part the full product of the Fischer-Tropsch reactionis separated into a light product stream, the light stream preferablycomprising all components boiling below the kero/diesel boiling range,and a heavy Fischer-Tropsch hydrocarbons stream, which stream is used instep (1). The light products stream comprises at least unreactedsynthesis gas, carbon dioxide, inert gasses as nitrogen and steam, andat least part of the hydrocarbons formed in the Fischer-Tropschreaction, preferably the C₁-C₁₀ hydrocarbons, preferably the C₁-C₄hydrocarbons. The heavy Fischer-Tropsch hydrocarbons stream comprises atleast all components boiling above the kero/diesel boiling range, butpreferably also the components boiling in the kero/diesel boiling range,as this improves the properties, especially the cold flow properties, ofthe product. Depending on the use of the product boiling below thekero/diesel boiling range, it may be advantageous or not to have itincorporated in the heavy Fischer-Tropsch stream. For instance, when itis the intention to use it as a component for gasoline, it is preferredto give it a hydrocracking/hydroisomerisation treatment to improve theoctane number. In the case that it is to be used as ethylene crackerfeedstock, it is preferred to avoid anyhydrocracking/hydroisomerisation.

Advantageously at least part of the effluent of theisomerisation/hydrocracking step is passed to a separation step in whicha hydrogen-containing gas and a hydrocarbon effluent are separated fromeach other. Suitably, in this separation step a hydrogen-containing gasand a hydrocarbon effluent are separated off by flash distillation.Suitably the flash distillation is carried out at a temperature between−20 and 100° C., and a pressure between 1 and 50 bar. Suitably thehydrocarbon fraction is separated into a fraction boiling above 370° C.and one or more fractions boiling below 370° C., e.g. two or threefractions boiling in the (light and heavy) gas oil range and a kerosenefraction. At least part of the heavy fraction obtained in the firsthydrocracking/hydroisomerisation reaction is introduced in the secondhydrocracking/hydroisomerisation reaction. Especially a substantial partof the 370° C. fraction is introduced in the second reaction, but alsosubstantial parts of the kerosene/gas oil fraction may be introducedinto this second step. Suitably at least 50 wt % of the 370° C. isintroduced into the second hydrocracking/hydroisomerisation step,preferably 70 wt %, more preferably at least 90 wt %, especially thetotal 370° C. plus fraction is introduced into the second step.

The conditions (catalyst, temperature, pressure, WHSV etc.) of thesecond hydrocracking/hydroisomerisation reaction are suitably similar tothe first reaction, although this is not necessarily the case. Theconditions and the preferred conditions are described above for thefirst reaction. In a preferred situation the conditions in the first andthe second hydrocracking/hydroisomerisation are the same.

Work-up of the products of the second hydrocracking/hydroisomerisationreaction is suitably similar to the first reaction (see above), althoughthis is not necessarily the case. In a preferred embodiment steps (2)and (4) are combined, i.e. the same distillation unit is used to producethe fuel products boiling in the kero/diesel range produced in steps (1)and (3).

At least part of the heavy fraction obtained in the secondhydrocracking/hydroisomerisation reaction is introduced in the first orsecond hydrocracking/hydroisomerisation reaction. Suitably at least 30wt % of the fraction boiling above 370° C. is introduced into the firsthydrocracking/hydroisomerisation step, preferably 60 wt %, morepreferably at least 90 wt %, especially the total 370° C. plus fractionis introduced into the second step. The remaining part of the fractionboiling above 370° C. may be used for different purposes, e.g. for thepreparation of base oils, but is preferably recycled to the firsthydrocracking/hydroisomerisation step.

In a preferred embodiment of the invention, the first and secondhydrocracking/hydroisomerisation reaction are combined into one reactionstep. This results in a very simple scheme, comprising onehydrocracking/isomerisation step and one separation step only. In thatcase at least part of the fraction boiling above 370° C. is recycled tothe combined hydrocracking/hydroisomerisation step, suitably at least 30wt %, preferably at least 60 wt %, more preferably at least 90 wt %. Theconversion per pass (of the fraction boiling above 370° C. (feed plusrecycle)) is suitably between 30 and 70 wt %, preferably between 40 and65 wt % (based on total feed supplied to thehydrocracking/hydroisomerisation step).

In a preferred embodiment of the present invention, the amount of heavyfraction obtained in step 2 which is used in step (3) or used in step(3) and recycled to step (1), is at least 70 wt %, preferably 85 wt %,more preferably 95 wt % of the total heavy fraction (i.e. boiling above370° C.). In another preferred embodiment the amount of heavy fractionobtained in step (4) which is used for step (1) and/or step (3), is atleast 70 wt %, preferably 85 wt %, more preferably 95 wt % of the totalheavy fraction.

The invention further relates to hydrocarbon products boiling on thekero/diesel boiling range obtainable by a process as defined above. Theinvention especially relates to a hydrocarbon fuel product, which hasnot been subjected to an additional dewaxing treatment, boiling in thediesel boiling range (defined above) having the following properties:cetane number at least 50, preferably at least 60, more preferably atleast 70, suitably up to 80, or even up to 90, iso/normal ratio between2.5 and 10, especially between 3.5 and 6, more especially between 4 and5, the amount of mono-iso compounds being at least 70 wt % (based ontotal product boiling in the diesel range), preferably 75 wt %, morepreferably 75-85%, cloud point below −10° C., preferably −20° C. (ingeneral up to −36° C.), CFPP below −20° C., preferably below −28° C. (ingeneral up to −44° C.) pour point below −15° C. and preferably below−22° C. (in general up to −40° C.). Preferably the hydrocarbon productas described above in which the amount of dimethyl compounds is between23 and 28 wt % (based on total product boiling in the diesel range). Theproducts obtained in step (4) of the process according to the presentinvention are preferred, as these products show extremely good cold flowproperties, i.e. cloud points below −26° C., CFPP below −30° C. and pourpoints below −24° C.

The invention is illustrated by the following non-limiting example.

EXAMPLE 1

A Fischer-Tropsch product was prepared in a process similar to theprocess as described in Example VII of WO-A-9934917 hereby incorporatedby reference, using the catalyst of Example III of WO-A-9934917 herebyincorporated by reference. The C₅+ fraction of the product thus obtainedwas continuously fed to a hydrocracking step (step (a)). The C₅+fraction contained about 60 wt % C30₊ product. The ratio C₆₀+/C₃₀+ wasabout 0.55. In the hydrocracking step the fraction was contacted with ahydrocracking catalyst of Example 1 of EP-A-532118 hereby incorporatedby reference. The effluent of step (a) was continuously distilled undervacuum to give light products, fuels and a residue “R” boiling from 370°C. and above. The conversion of the product boiling above 370° C. intoproduct boiling below 370° C. was between 45 and 55 wt %. The residue“R” was recycled to step (a). The conditions in the hydrocracking step(a) were: a fresh feed Weight Hourly Space Velocity (WHSV) of 0.8kg/l.h, recycle feed WHSV of 0.4 kg/l.h, hydrogen gas rate=1000 Nl/kg,total pressure=40 bar, and a reactor temperature of 330° C., 335° C. or340° C. A comparison example was carried out with Fischer Tropschmaterial made with a cobalt/zirconia/silica catalyst as described in EP426223 hereby incorporated by reference using conditions similar to theconditions as described above. The C₅+ fraction contained about 30 wt %C₃₀+ product, the ratio C₆₀+/C₃₀+ was 0.19. The properties of the dieselfuel fractions are summarized in the Table. Experiments I, II and IIIare according to the invention, Experiments IV and V are comparisonexperiments. The temperatures mentioned in the Table are thetemperatures of the hydrocracking step. Cloud point, Pour point and CFPPwere determined by ASTM D2500, ASTM D97 and IP 309-96. Establishment ofthe C₅+, C₃₀+ and C₆₀+ fractions were done by gas chromatography.

TABLE Experiment I II III IV V Temperature 330 335 340 330 335 CloudPoint −13 −20 <−24 +1 − 2 CFPP −14 −21 −28 0 −5 Pour Point −18 <−24 <−240 −6 Normals (wt %) 27.6 21.3 19.9 50.4 41.2 Iso's (wt %) 72.4 78.7 80.149.6 58.8 Mono-methyl 37.3 39.5 39.5 29.2 32.2 Di-methyl 21.7 25.5 26.713.9 18.1 Others 13.4 13.8 14.1 6.4 8.5 Density (kg/l) 0.78 0.78 0.780.78 0.78 Cetane (D976m) 78 77 76 80 78 Cetane (D4737m) 87 85 86 90 85T95 363 360 358 — —

1. A process for the preparation of one or more hydrocarbon fuelproducts boiling in the kero/diesel range from a stream of hydrocarbonsproduced in a Fischer Tropsch process comprising converting synthesisgas into liquid hydrocarbons, at least a part of the hydrocarbonsboiling above the kero/diesel range, comprising the following steps: (1)hydrocracking/hydroisomerizing at least a part of the Fischer Tropschhydrocarbons stream at a conversion per pass of at most 80 wt % of thematerial boiling above 370° C. into material boiling below 370° C.; (2)separating the product stream obtained in step (1) into one or morelight fractions boiling below the kero/diesel boiling range, one or morefractions boiling in the kero/diesel boiling range and a heavy fractionboiling above the kero/diesel boiling range; (3)hydrocracking/hydroisomerizing a major part of the heavy fractionobtained in step (2) at a conversion per pass of at most 80 wt % of thematerial boiling above 370° C. into a product stream of material boilingbelow 370° C.; (4) separating the product stream obtained in step (3)into one or more light fractions boiling below the kero/diesel boilingrange, one or more fractions boiling in the kero/diesel boiling rangeand a heavy fraction boiling above the kero/diesel boiling range; and,(5) hydrocracking/hydroisomerizing a major part of the heavy fractionobtained in step (4) in the hydrocracking/hydroisomerizing processdescribed in step (1) and/or step (3), in which process the FischerTropsch hydrocarbons stream comprises at least 35 wt % C₃₀+ (based ontotal amount of hydrocarbons in the Fischer Tropsch hydrocarbons stream)and in which stream a weight ratio C₃₀+/C₆₀+ is at least 0.2.
 2. Theprocess of claim 1, wherein the Fischer Tropsch process furthercomprises converting synthesis gas into liquid hydrocarbons over an ironor cobalt catalyst.
 3. The process of claim 2, wherein the catalystcomprises a cobalt catalyst comprising a carrier; and, optionally one ormore promoters selected from the group consisting of vanadium,manganese, rhenium, zirconium and platinum.
 4. The process of claim 1,wherein the Fischer Tropsch process further comprises conditions suchthat the Anderson-Schulz-Flory alpha value for the obtained productshaving at least 20 carbon atoms is at least 0.925.
 5. The process ofclaim 1, wherein the Fischer Tropsch process comprises a slurry FischerTropsch process or a fixed bed Fischer Tropsch process.
 6. The processof claim 1, wherein at least part of the full product of the FischerTropsch reaction is separated into a light product stream,,; and, aheavy Fischer Tropsch hydrocarbons stream, which stream is used in step(1).
 7. The process of claim 6, wherein the light products streamcomprises unreacted synthesis gas, carbon dioxide, inert gases such asnitrogen and steam, and C₁-C₄ hydrocarbons.
 8. The process of claim 1wherein the Fischer Tropsch hydrocarbons stream comprises at least 40 wt% C₃₀+ hydrocarbons, based on total hydrocarbons stream.
 9. The processof claim 1 wherein the product boiling in the kero/diesel boiling rangehas a boiling range within the range of 110° C. and 400° C.
 10. Theprocess of claim 1 wherein the conversion per pass in steps (1) and/or(3) of the material boiling above 370° C. into material boiling below370° C. is between 30 wt % and 70 wt.
 11. The process of claim 1 whereinthe first hydrocracking/hydroisomerization step is carried out at atemperature between 290° C. and 375° C., a pressure between 15 and 200bar and a WHSV between 0.5 and 3 kg/l/h.
 12. The process of claim 1wherein the second hydrocracking/hydroisomerisation step is carried outat a temperature between 290° C. and 375° C., a pressure between 15 and200 bar, and a WHSV between 0.5 and 3 kg/l/h.
 13. The process of claim12, wherein in the second hydrocracking/hydroisomerization step the sameconditions are used as in the first hydrocracking/hydroisomerizationstep.
 14. The process of claim 1 wherein a part of the heavy boilingfraction obtained in step (2) which fraction is not introduced in theprocess of step (3), is recycled to step (1).
 15. The process of claim13, wherein the first and the second hydrocracking/hydroisomerizationstep are combined in the same reactor.
 16. The process of claim 1wherein the amount of heavy fraction obtained in step (2) which is usedin step (3) or used in step (3) and recycled to step (1), is at least 70wt of the total heavy fraction.
 17. The process of claim 1 wherein theamount of heavy fraction obtained in step (4) which is used for step (2)in step (1) and/or step (3), is at least 70 wt % of the total heavyfraction.